Reserves for Unpaid Losses and Loss Adjustment Expenses Reserves for Unpaid Losses and Loss Adjustment Expenses (Reserve Development) (Details) - USD ($) $ in Millions		3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year				$ 712.9	$ 732.0	$ 622.1
Prior Accident Year				(4.0)	92.0	0.0
Total incurred losses and LAE				708.9	824.0	$ 622.1
OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		$ 33.7	$ 33.7
Prior Accident Year	$ 75.5	75.5	14.3		75.5
Total incurred losses and LAE		109.2	$ 109.2	$ 1.8	89.8
Professional Insurance | OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		22.9
Prior Accident Year		46.4
Total incurred losses and LAE		69.3			59.1
Specialty Property | OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		(1.1)
Prior Accident Year		5.7
Total incurred losses and LAE		4.6			1.1
Crop | OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		3.8
Prior Accident Year		0.0
Total incurred losses and LAE		3.8			0.0
Other | OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		2.8
Prior Accident Year		(0.4)
Total incurred losses and LAE		2.4			1.6
Specialty Products | OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		28.4
Prior Accident Year		51.7
Total incurred losses and LAE		80.1			61.8
Entertainment | OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		1.5
Prior Accident Year		11.6
Total incurred losses and LAE		13.1			13.5
Accident | OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		0.0
Prior Accident Year		3.5
Total incurred losses and LAE		3.5			6.0
Government Risks | OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		1.2
Prior Accident Year		7.1
Total incurred losses and LAE		8.3			8.5
Other | OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		2.6
Prior Accident Year		1.6
Total incurred losses and LAE		4.2			0.0
Specialty Industries | OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		5.3
Prior Accident Year		23.8
Total incurred losses and LAE		29.1			$ 28.0
Management's estimate [Member] | Specialty Products | OneBeacon
Loss and Loss Adjustment, Expense Reserves [Line Items]
Current Accident Year		$ 29.9